3,788 research outputs found
Life Products of Stars
We attempt to document complete energetic transactions of stars in their
life. We calculate photon and neutrino energies that are produced from stars in
their each phase of evolution from 1 to 8 M_sun, using the state-of-the-art
stellar evolution code, tracing the evolution continuously from pre-main
sequence gravitational contraction to white dwarfs. We also catalogue
gravitational and thermal energies and helium, and heavier elements that are
stored in stars and those ejected into interstellar space in each evolutionary
phase.Comment: 26 pages, including 8 figures and 3 tables. Submitted to ApJ
On the degenerated soft-mode instability
We consider instabilities of a single mode with finite wavenumber in
inversion symmetric spatially one dimensional systems, where the character of
the bifurcation changes from sub- to supercritical behaviour. Starting from a
general equation of motion the full amplitude equation is derived
systematically and formulas for the dependence of the coefficients on the
system parameters are obtained. We emphasise the importance of nonlinear
derivative terms in the amplitude equation for the behaviour in the vicinity of
the bifurcation point. Especially the numerical values of the corresponding
coefficients determine the region of coexistence between the stable trivial
solution and stable spatially periodic patterns. Our approach clearly shows
that similar considerations fail for the case of oscillatory instabilities.Comment: 16 pages, uses iop style files, manuscript also available at
ftp://athene.fkp.physik.th-darmstadt.de/pub/publications/wolfram/jpa_97/ or
at http://athene.fkp.physik.th-darmstadt.de/public/wolfram_publ.html. J.
Phys. A in pres
Functional requirements for onboard management of space shuttle consumables, volume 1
A study was conducted to determine the functional requirements for onboard management of space shuttle consumables. A generalized consumable management concept was developed for application to advanced spacecraft. The subsystems and related consumables selected for inclusion in the consumables management system are: (1) propulsion, (2) power generation, and (3) environmental and life support
Convectiveâreactive nucleosynthesis of K, Sc, Cl and p-process isotopes in OâC shell mergers
© 2017 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. We address the deficiency of odd-Z elements P, Cl, K and Sc in Galactic chemical evolution models through an investigation of the nucleosynthesis of interacting convective O and C shells in massive stars. 3D hydrodynamic simulations of O-shell convection with moderate C-ingestion rates show no dramatic deviation from spherical symmetry. We derive a spherically averaged diffusion coefficient for 1D nucleosynthesis simulations, which show that such convective-reactive ingestion events can be a production site for P, Cl, K and Sc. An entrainment rate of 10-3Mâs-1features overproduction factors OPsâ 7. Full O-C shell mergers in our 1D stellar evolution massive star models have overproduction factors OPm> 1 dex but for such cases 3D hydrodynamic simulations suggest deviations from spherical symmetry. Îł - process species can be produced with overproduction factors of OPm> 1 dex, for example, for130, 132Ba. Using the uncertain prediction of the 15Mâ, Z = 0.02 massive star model (OPmâ 15) as representative for merger or entrainment convective-reactive events involving O- and C-burning shells, and assume that such events occur in more than 50 per cent of all stars, our chemical evolution models reproduce the observed Galactic trends of the odd-Z elements
The s-Process in Rotating Asymptotic Giant Branch Stars
(abridged) We model the nucleosynthesis during the thermal pulse phase of a
rotating, solar metallicity AGB star of 3M_sun. Rotationally induced mixing
during the thermal pulses produces a layer (~2E-5M_sun) on top of the CO-core
where large amounts of protons and C12 co-exist. We follow the abundance
evolution in this layer, in particular that of the neutron source C13 and of
the neutron poison N14. In our AGB model mixing persists during the entire
interpulse phase due to the steep angular velocity gradient at the
core-envelope interface. We follow the neutron production during the interpulse
phase, and find a resulting maximum neutron exposure of tau_max =0.04 mbarn^-1,
which is too small to produce any significant s-process. In parametric models,
we then investigate the combined effects of diffusive overshooting from the
convective envelope and rotationally induced mixing. Models with overshoot and
weaker interpulse mixing - as perhaps expected from more slowly rotating stars
- yield larger neutron exposures. We conclude that the incorporation of
rotationally induce mixing processes has important consequences for the
production of heavy elements in AGB stars. Through a distribution of initial
rotation rates it may lead to a natural spread in the neutron exposures
obtained in AGB stars of a given mass - as appears to be required by
observations. Our results suggest that both processes, diffusive overshoot and
rotational mixing, may be required to obtain a consistent description of the
s-process in AGB stars which fulfils all observational constraints. Finally, we
find that mixing due to rotation within our current framework does increase the
production of N15 in the partial mixing zone, however still falling short of
what seems required by observations.Comment: 50 pages, 13 figures, ApJ in press, tentatively scheduled for v593 n2
August 20, 200
The Bioeconomics of Controlling an African Rodent Pest Species
The paper treats the economy of controlling an African pest rodent, the multimammate rat, causing major damage in maize production. An ecological population model is presented and used as a basis for the economic analyses carried out at the village level using data from Tanzania. This model incorporates both density-dependent and density-independent (stochastic) factors. Rodents are controlled by applying poison, and the economic benefits depend on the income from maize production minus the costs for maize production, fertiliser and poison. We analyse how the net present value of maize production is affected by various rodent control strategies, by varying the duration and timing of rodenticide application. Our numerical results suggest that, in association with fertiliser, it is economically beneficial to control the rodent population. In general the most rewarding duration of controlling the rodent population is 3-4 months every year, and especially at the end of the dry season/beginning of rainy season. The paper demonstrates that changing from todays practice of symptomatic treatment when heavy rodent damage is noticed to a practice where the calendar is emphasised, may substantially improve the economic conditions for the maize producing farmers. This main conclusion is quite robust and not much affected by changing prices and costs of the maize production.bio-economics; pest control; multimammate rat; crop production
The onset of photoionization in Sakurai's Object (V4334 Sgr)
We investigate the reheating of the very late thermal pulse (VLTP) object
V4334 Sgr (Sakurai's Object) using radio observations from the Very Large
Array, and optical spectra obtained with the Very Large Telescope. We find a
sudden rise of the radio flux at 5 and 8 GHz - from <= 90 micro-Jy and 80 +/-
30 micro-Jy in February 2005 to 320 micro-Jy and 280 micro-Jy in June 2006.
Optical line emission is also evolving, but the emission lines are fading. The
optical line emission and early radio flux are attributed to a fast shock (and
not photoionization as was reported earlier) which occurred around 1998. The
fading is due to post-shock cooling and recombination. The recent rapid
increase in radio flux is evidence for the onset of photoionization of carbon
starting around 2005. The current results indicate an increase in the stellar
temperature to 12 kK in 2006. The mass ejected in the VLTP eruption is M_ej >=
1e-4 Msol, but could be as high as 1e-2 Msol, depending mainly on the distance
and the clumping factor of the outflow. We derive a distance between 1.8 and 5
kpc. A high mass loss could expose the helium layer and yield abundances
compatible with those of [WC] and PG1159 stars.Comment: 4 pages, 2 figures; accepted for publication in A&A letter
The Abundance Evolution of Oxygen, Sodium and Magnesium in Extremely Metal-Poor Intermediate Mass Stars: Implications for the Self-Polution Scenario in Globular Clusters
We present full stellar evolution and parametric models of the surface
abundance evolution of O16, Ne22, Na23 and the magnesium isotopes in an
extremely metal-poor intermediate mass star M_ZAMS=5M_sun, Z=0.0001. O16 and
Ne22 are injected into the envelope by the third dredge-up following thermal
pulses on the asymptotic giant branch. These species and the initially present
Mg24 are depleted by hot bottom burning (HBB) during the interpulse phase. As a
result, Na23, Mg25 and Mg26 are enhanced. If the HBB temperatures are
sufficiently high for this process to deplete oxygen efficiently, Na23 is first
produced and then depleted during the interpulse phase. Although the
simultaneous depletion of O16 and enhancement of Na23 is possible, the required
fine tuning of the dredge-up and HBB casts some doubt on the robustness of this
process as the origin of the O-Na anti-correlation observed in globular cluster
stars. However, a very robust prediction of our models are low Mg24/Mg25 and
Mg24/Mg26 ratios whenever significant O16 depletion can be achieved. This seems
to be in stark contrast with recent observations of the magnesium isotopic
ratios in the globular cluster NGC6752.Comment: ApJ Letters, in pres
The Nature of the Driving Mechanism in the Pulsating Hybrid PG 1159 Star Abell 43
We extend our previous pulsational stability analyses of PG 1159 stars by
modeling the hybrid PG 1159 type star Abell 43. We show that the standard
kappa-mechanism due to the ionization of C and O in the envelope of this H-rich
PG 1159 star is perfectly able to drive g-mode pulsations. Thus, contrary to a
recent suggestion, there is no need to invoke any new or exotic mechanism to
explain the pulsational instabilities observed in this particular star. Our
expected instability band for l = 1 modes extends in period from ~ 2604 s to ~
5529 s, which is consistent with the available photometric observations of
Abell 43. We also suggest that efforts to detect luminosity variations in its
sibling NGC 7094 be pursued.Comment: 3 pages, 1 figure, Accepted for publication in A&
- âŠ